This invention relates to a balance control system for multichannel audio apparatus and, in particular, to an improved balance control system wherein both amplitudes and times of arrival at a given point stereophonic reproduced sterophonic sounds are balanced.
In stereophonic sound reproducing apparatus, particularly of the type designated for entertainment purposes, desirable sound characteristics will be sensed by a listener who is positioned at a generally central location with respect to the sound transducers. For example, if the stereophonic sounds are reproduced by two conventional loudspeaker systems, the listener generally will be located at a point which is substantially equidistant from both loudspeaker systems. At that point, optimum sound sensation is perceived if the sounds produced by the left and right loudspeaker systems, for example, are of equal amplitude. Since the listener is equidistant from each loudspeaker system, the propagation time required for the respective reproduced sounds to reach the listener's location is equal and the arriving sound waves will admit of the proper relative phase. Now, if the listener changes his position with respect to the loudspeaker systems, the relative sound characteristics transmitted to his new location should be adjusted with respect to each other to compensate for the change in position. This adjustment is performed by a balance control circuit which usually is provided in the stereophonic signal reproducing apparatus in an attempt to restore to the listener the same sound sensations that had previously been sensed, notwithstanding his change in location. For example, if the listener is now located closer to the left loudspeader system than to the right loudspeaker system, the magnitude of the sound propagated from the left loudspeaker system should be reduced, while the magnitude of the sound propagated from the right loudspeaker system should be increased. The conventional balance control circuit attains this balance in the propagated sounds by reducing the audio signal transmitted over the left signal channel while increasing the audio signal transmitted over the right signal channel. It is expected that, since the left channel sound signals are reduced with respect to the right channel sound signals, but the listener is closer to the left loudspeaker system, then the greater propagation attenuation of the right channel sound signals will result in the arrival at the listener's location of sounds admitting of the same relative magnitude as when the listener had previously been located at the equidistant position.
Unfortunately, it has been found that, since the listener is now located closer to one loudspeaker system than to the other, the propagation times required for the respective sounds to arrive at the listener's position are no longer equal. That is, consistent with the foregoing example, the sound wave emitted from the left loudspeaker system arrives at the listener's location in advance of the sound wave emitted from the right loudspeaker system. Therefore, although the sound volume is in balance, the impinging sound waves are out of phase with respect to each other, thereby having a deleterious effect upon the received acoustic image.